Chikungunya virus (CHIKV) and Ross River virus (RRV) are mosquito-transmitted arthritogenic alphaviruses that cause large epidemics of debilitating inflammatory musculoskeletal disease. Previous studies by our group revealed that substitution of the gene for the non-structural protein 1 (nsP1) from an attenuated strain of RRV (DC5692) into the T48 strain was sufficient to attenuate the previously virulent virus. This effect was mapped to two specific amino acid changes in nsP1: S79C and L224I. Within this proposal, I aim to define host mechanisms responsible for attenuation of this mutant virus and, more broadly, the mechanisms driving innate immune control of these alphaviruses. While monocytes and their derived cells have been implicated in the pathogenesis of these infections, the precise roles these cells play during alphavirus infections has not yet been clearly defined. Preliminary experiments have shown that CCR2+ inflammatory monocytes are critical to the control of the attenuated mutant as well as wild-type RRV and CHIKV. Based on this, I hypothesize that monocytes mediate control of acute alphavirus infection through a type I IFN-dependent mechanism. This hypothesis will be tested in two specific aims: 1) to define the mechanism(s) by which inflammatory monocytes control acute alphavirus infection and 2) to define the role of IL-1 in acute alphavirus infection. Given that type I IFN is critical to control of alphavirus infection, and the nsP1 mutant virus shows increased sensitivity to type I IFN in vitro and is full virulent in Ifnar1-/- mice as well as that monocytes are essential to control of infection, I hypothesize that tissue-infiltrating monocytes, activated via TLR stimulation, control acute alphavirus infection via local production of type I IFN. I will test this using FACS sorting of monocytes from infected mice to look at the induction and activation of IRFs as well as adoptive transfer of monocytes genetically deficient in TLR3 or IRF3 and IRF7 into mice depleted of monocytes to assess the effects of these pathways on monocyte-mediated control. Also, there has been recent evidence to suggest a role for IL-1 in regulation of type I IFN signaling in several contexts, and my preliminary studies have found that IL-1? antagonizes the anti-RRV effect of IFN? treatment in vitro and that the nsP1 mutant induces significantly less IL-1 in viv in comparison to mice infected with wild-type RRV. Thus, I hypothesize that high expression levels of IL-1 antagonizes type I IFN-mediated control of acute RRV and CHIKV infection. I will test this using a series of in vitro and in vivo experiments to assess the virus and cell type specificity of the effects of IL-1 as well as whether the loss or blockade of IL-1 signaling improves host control of acute alphavirus infection in mice.